1. Field of the Invention
This invention relates to processes for depositing films onto substrates by reactive magnetron sputtering. More specifically it relates to deposition of these films at high rates over large areas at low temperatures.
2. Description of the Background Art
The publications and other reference materials referred to herein to describe the background of the invention and to provide additional detail regarding its practice are hereby incorporated by reference. For convenience, the reference materials are numerically referenced and grouped in the appended bibliography.
Transparent conducting films have been extensively used in a variety of electronic and optoelectronic industries..sup.(1) Stringent requirements on optical transparency (i.e. greater than 85% transmission) and electrical resistivity (.rho. of approximately 10.sup.-4 ohm-cm) for many specific optoelectronic applications has led to considerable research in the past decade towards development of new materials based on non-stoichiometric oxides of tin, indium, cadmium, zinc and their alloys as well as suitable techniques for synthesizing such coatings.
Many techniques such as chemical vapor deposition.sup.(2), spray pyrolysis.sup.(3), reactive.sup.(4) as well as activated reactive evaporation.sup.(5), sputtering.sup.(6), and reactive sputtering.sup.(7) have been used. Although many of the above techniques have produced excellent films, a suitable process for synthesis of such films onto large area substrates at relatively low substrate temperatures has not been developed.
Due to difficulties encountered in sputtering from an oxide target, such as physical cracking of the target, inability to control target surface composition, etc., a simple process based on reactive magnetron sputtering from a metallic target has been explored by a number of researchers. Although conventional reactive magnetron sputtering has been used successfully to deposit transparent conducting films of indium tin Oxide, difficulties are encountered in this process as regards to the deposition rate and run-to-run reproducibility of the film properties. The above difficulties stem from the inevitable "poisoning" of the target due to the formation of an oxide layer on the target's surface during reactive sputtering of the respective metal in the oxygen containing plasma.
The "poisoning" of the target depends in complex manner on deposition parameters such as target power, flow rate and partial pressure of the reactive gas, etc..sup.(8,9,10). The narrow operating window within which the target can be maintained in an unoxidized surface condition and yet deposit an oxide coating makes it extremely difficult to adopt conventional reactive magnetron sputtering for the synthesis of oxide films for industrial applications.
A number of modifications have been proposed to alleviate the above problems associated with the "poisoning" of the target during reactive magnetron sputtering from metallic targets. All such modifications are aimed at reducing the oxygen partial pressure in the vicinity of the target by using gettering surfaces.sup.(11) and/or geometrical baffles around the target.sup.(12). The use of an r.f. substrate bias in conjunction with the above arrangements has also been explored..sup.(13,14).
Although the above modifications have proven to be successful in reducing the target poisoning, they suffer from limitations as regards to their application for deposition onto large area substrates. The major limitations are: (1) Poor target utilization--only a fraction of the material is deposited on the substrate. (2) The use of geometric baffles limits the area of the substrate that is coated. (3) In most of the above configurations, target/substrates distances are very small: 3-5 inches. This leads to difficulties in obtaining thickness uniformity over large substrate areas. Also, a small target/substrate spacing imposes limitations when complex shape substrates need to be coated.
In view of the above limitations of the existing reactive magnetron sputtering technique for synthesis of transparent conducting coatings, there is presently a need to provide further changes in the basic process so as to establish a suitable technique that can be used for the synthesis of good quality transparent conducting coatings onto large area, heat sensitive substrates for industrial applications. For example, a heat sensitive substrate is a polymer which degrades at relatively low temperatures, such as 80.degree. C.